workflows: Fix unit-tests' "changed" detection to compare against correct target branch
workflows: Fix unit-tests' "changed" detection to compare against correct target branch

Hardcoding "master" does not work for PRs which target e.g. rhel-8.4.

martinpitt/cockpit,martinpitt/cockpit,cockpit-project/cockpit,martinpitt/cockpit,mvollmer/cockpit,cockpit-project/cockpit,martinpitt/cockpit,mvollmer/cockpit,cockpit-project/cockpit,mvollmer/cockpit,martinpitt/cockpit,mvollmer/cockpit,mvollmer/cockpit,cockpit-project/cockpit,cockpit-project/cockpit